Methods for controlling towed marine sensor array geometry

ABSTRACT

A method for towing a streamer array includes moving a vessel along a body of water. Streamers are towed by vessel. A relative position is determined at selected points along each streamer with respect to the vessel. At least one of the streamers is deflected at at least one longitudinal position along the streamer in response to the determined positions to maintain the streamers in a selected geometry. The selected geometry is related to one of survey vessel heading, energy source trajectory, previously plotted sensor trajectory and a lateral separation related to distance from the towing vessel.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-In-Part of U.S. patentapplication Ser. No. 12/148,610 filed on Apr. 21, 2008.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of marine geophysicalsurveying. More particularly, the invention relates to methods forcontrolling the spatial distribution or geometry of an array ofgeophysical sensor streamers towed behind a survey vessel.

2. Background Art

Marine geophysical surveying systems such as seismic acquisition systemsand electromagnetic survey systems are used to acquire geophysical datafrom formations disposed below the bottom of a body of water, such as alake or the ocean. Marine seismic surveying systems, for example,typically include a seismic survey vessel having onboard navigation,seismic energy source control, and geophysical data recording equipment.The seismic survey vessel is typically configured to tow one or more(typically a plurality) of laterally spaced apart sensor streamersthrough the water. At selected times, the seismic energy source controlequipment causes one or more seismic energy sources (which may be towedin the water by the seismic vessel or by another vessel) to actuate.Signals generated by various sensors on the one or more streamers inresponse to detected seismic energy are ultimately conducted to therecording equipment. A record with respect to time is made in therecording system of the signals generated by each sensor (or groups ofsuch sensors). The recorded signals are later interpreted to infer thestructure and composition of the formations below the bottom of the bodyof water. Corresponding components for inducing electromagnetic fieldsand detecting electromagnetic phenomena originating in the subsurface inresponse to such imparted fields may be used in marine electromagneticgeophysical survey systems.

The one or more sensor streamers are in the most general sense longcables that have geophysical sensors disposed at spaced apart positionsalong the length of the cables. A typical streamer can extend behind thegeophysical survey vessel for several kilometers. Because of the greatlength of the typical streamer, the streamer may not travel entirely ina straight line behind the survey vessel at every point along its lengthdue to interaction of the streamer with the water and currents in thewater, among other factors.

Streamers towed by a vessel configured for towing multiple streamers areassociated with equipment that maintain the forward ends of thestreamers at selected lateral distances from each other and from thecenterline of the survey vessel as they are towed through the water.Such multiple streamer systems are used in what are known as threedimensional and four dimensional geophysical surveys. A four dimensionalseismic survey is a three dimensional survey over a same area of theEarth's subsurface repeated at selected times. The individual streamersin such systems are affected by the same forces that affect a singlestreamer.

The quality of geophysical images of the Earth's subsurface producedfrom three dimensional surveys is affected by how well the positions ofthe individual sensors on the streamers are controlled. The quality ofimages generated from the detected signals also depends to an extent onthe relative positions of the sensors being maintained throughout thegeophysical survey. Various devices are known in the art for positioningstreamers laterally and/or at a selected depth below the water surface.U.S. Pat. No. 5,443,027 issued to Owsley et al., for example, describesa lateral force device for displacing a towed underwater acoustic cablethat provides displacement in the horizontal and vertical directions.The device has a hollow spool and a rotationally mounted wingedfuselage. The hollow spool is mounted on a cable with cable elementspassing therethrough. The winged fuselage is made with the top halfrelatively positively buoyant and the bottom half relatively negativelybuoyant. The winged fuselage is mounted about the hollow spool withclearance to allow rotation of the winged fuselage. The difference inbuoyancy between the upper and lower fuselage maintains the device inthe correct operating position. Wings on the fuselage are angled toprovide lift in the desired direction as the winged fuselage is towedthrough the water. The device disclosed in the Owsley et al. patentprovides no active control of direction or depth of the streamer,however.

U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a seismicstreamer position control module having a body with a first end and asecond end and a bore therethrough from the first end to the second endfor receiving a seismic streamer therethrough. The module has at leastone control surface, and at least one recess in which is initiallydisposed the at least one control surface. The at least one controlsurface is movably connected to the body for movement from and into theat least one recess and for movement, when extended from the body, forattitude adjustment. Generally, the device described in the Ambs et al.patent is somewhat larger diameter, even when closed, than the streamerto which it is affixed, and such diameter may become an issue whendeploying and retrieving streamers from the water.

U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a method forcontrolling the navigation of a towed seismic streamer using “birds”affixable to the exterior of the streamer. The birds are equipped withvariable-incidence wings and are rotatably fixed onto the streamer.Through a differential action, the wings allow the birds to be turnedabout the longitudinal axis of the streamer so that a hydrodynamic forceoriented in any given direction about the longitudinal axis of thestreamer is obtained. Power and control signals are transmitted betweenthe streamer and the bird by rotary transformers. The bird is fixed tothe streamer by a bore closed by a cover. The bird can be detachedautomatically as the streamer is raised so that the streamer can bewound freely onto a drum. The disclosed method purportedly allows thefull control of the deformation, immersion and heading of the streamer.

UK Patent 2,364,388, by Canter et al. discloses a method of repeating amarine seismic survey of a sub-surface area that has been previouslysurveyed.

U.S. Pat. Nos. 6,932,017; 7,080,607; 7,222,579 and 7,293,520 (all baseddirectly or indirectly on PCT Application PCT/IB99/01590) describevarious aspects of a control system for positioning of marine seismicstreamers.

SUMMARY OF THE INVENTION

A method for towing a sensor streamer array in a body of water accordingto one aspect of the invention includes towing a plurality of sensorstreamers behind a survey vessel in the water. A relative position isdetermined at selected points along each streamer with respect to thevessel. At least one streamer is deflected at at least one longitudinalposition therealong in response to the determined positions such thatthe streamers are maintained in a selected geometry in the water, inwhich positions along each streamer substantially follow a geodeticheading of the survey vessel.

A method for towing a sensor streamer array in a body of water accordingto another aspect of the invention includes towing a plurality of sensorstreamers behind a survey vessel. A relative position is determined atselected points along each streamer with respect to the vessel. At leastone streamer is deflected at at least one longitudinal positiontherealong in response to the determined positions such that thestreamers are maintained in a selected geometry in the water, in whichdetermined positions along each streamer substantially follow a geodetictrajectory of a geophysical energy source towed in the water.

A method for towing a sensor streamer array in a body of water accordingto another aspect of the invention includes towing a plurality of sensorstreamers behind a survey vessel. A relative position is determined atselected points along each streamer with respect to the vessel. At leastone streamer is deflected at at least one longitudinal positiontherealong in response to the determined positions such that thestreamers are maintained in a selected geometry in the water, in whichdetermined positions along each streamer substantially follow a selectedgeodetic path.

A method for towing a sensor streamer array in a body of water accordingto another aspect of the invention includes towing a plurality of sensorstreamers behind a survey vessel. A lateral deflecting force is appliedat selected positions along each streamer. The lateral deflecting forceat each selected position is proportional to a lateral distance of aforward end of each streamer from a center line of the survey vessel.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an array of seismic streamers each including lateral forceand depth control devices for adjusting geometry of the respectivestreamer.

FIGS. 2 through 4 schematically show various examples of controllingarray geometry according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows a typical marine geophysical survey system that can includea plurality of sensor streamers. Each of the sensor streamers can beguided through the water by one or more lateral force and depth (“LFD”)control devices cooperatively engaged with each of the streamers. Aswill be explained further below, the use of LFD devices, which providedepth control capability in addition to horizontal position control, isa matter of choice. It is only necessary for purposes of the inventionthat the devices associated with the sensor streamers providedirectional control, that is, to affect the direction of the streamerparallel to the plane of the water surface as it moves through thewater. The geophysical survey system includes a survey vessel 10 thatmoves along the surface of a body of water 11 such as a lake or theocean. The survey vessel 10 may include thereon equipment, showngenerally at 12 and for convenience collectively referred to as a“recording system.” The recording system 12 typically includes devicessuch as a data recording unit (not shown separately) for making a recordwith respect to time of signals generated by various sensors in theacquisition system. The recording system 12 also typically includesnavigation equipment (not shown separately) to determine and record, atselected times, the geodetic position of the vessel 10, and using otherdevices to be explained below, each of a plurality of geophysicalsensors 22 disposed at spaced apart locations on streamers 20 towed bythe survey vessel 10.

In one example, the device for determining the geodetic position may bea geodetic position signal receiver 12A such as a global positioningsystem (“GPS”) receiver, shown schematically at 12A. Other geodeticposition determination devices are known in the art. The foregoingelements of the recording system 12 are familiar to those skilled in theart, and with the exception of the geodetic position detecting receiver12A, are not shown separately in the figures herein for clarity of theillustration.

The geophysical sensors 22 can be any type of geophysical sensor knownin the art. Non-limiting examples of such sensors may include particlemotion-responsive seismic sensors such as geophones and accelerometers,pressure-responsive seismic sensors, pressure time gradient-responsiveseismic sensors, electrodes, magnetometers, temperature sensors orcombinations of the foregoing. In various implementations of theinvention, the geophysical sensors 22 may measure, for example, seismicor electromagnetic field energy primarily reflected from or refracted byvarious structures in the Earth's subsurface below the bottom of thewater 11 in response to energy imparted into the subsurface by an energysource 17. Seismic energy, for example, may originate from a seismicenergy source, or an array of such sources, deployed in the water 11 andtowed by the survey vessel 10 or by another vessel. Electromagneticenergy may be provided by passing electric current through a wire loopor electrode pair (not shown for clarity). The energy source (not shown)may be towed in the water 11 by the survey vessel 10 or a differentvessel (not shown). The recording system 12 may also include energysource control equipment (not shown separately) for selectivelyoperating the energy source 17.

In the survey system shown in FIG. 1, there are four sensor streamers 20towed by the survey vessel 10. The number of sensor streamers shown inFIG. 1, however, is only for purposes of explaining the invention and isnot a limitation on the number of streamers that may be used in anyparticular geophysical survey system according to the invention. Asexplained in the Background section herein, in marine geophysicalacquisition systems such as shown in FIG. 1 that include a plurality oflaterally spaced apart streamers, the streamers 20 are typically coupledto towing equipment that secures the forward end of each of thestreamers 20 at a selected lateral position with respect to adjacentstreamers and with respect to the seismic vessel 10. As shown in FIG. 1,the towing equipment can include two paravane tow ropes 8 each coupledto the vessel 10 at one end through a winch 19 or similar spoolingdevice that enables changing the deployed length of each paravane towrope 8. The distal end of each paravane tow rope 8 is functionallycoupled to a paravane 14. The paravanes 14 are each shaped to provide alateral component of motion to the various towing components deployed inthe water 11 when the paravanes 14 are moved through the water 11.“Lateral” in the present context means transverse to the direction ofmotion of the survey vessel 10 in the water 11. The lateral motioncomponent of each paravane 14 is opposed to that of the other paravane14. The combined lateral motion component of the paravanes 14 separatesthe paravanes 14 from each other until they put into tension one or morespreader ropes or cables 24, functionally coupled end to end between theparavanes 14.

The sensor streamers 20 can each be coupled, at the axial end thereofnearest the vessel 10 (the “forward end”), to a respective lead-in cabletermination 20A. The lead-in cable terminations 20A can be coupled to orassociated with the spreader ropes or cables 24 so as to fix the lateralpositions of the streamers 20 with respect to each other and withrespect to the centerline of the vessel 10. Electrical and/or opticalconnection between the appropriate components in the recording system 12and, ultimately, the geophysical sensors 22 (and/or other circuitry) inthe ones of the streamers 20 inward of the lateral edges of the systemmay be made using inner lead-in cables 18, each of which terminates in arespective lead-in cable termination 20A. A lead-in termination 20A isdisposed at the forward end of each streamer 20. Correspondingelectrical and/or optical connection between the appropriate componentsof the recording system 12 and the sensors 22 in the laterally outermoststreamers 20 may be made through respective lead-in terminations 20A,using outermost lead-in cables 16. Each of the inner lead-in cables 18and outermost lead-in cables 16 may be deployed by a respective winch 19or similar spooling device such that the deployed length of each cable16, 18 can be changed. The type of towing equipment coupled to theforward end of each streamer shown in FIG. 1 is only intended toillustrate a type of equipment that can tow an array of laterally spacedapart streamers in the water. Other towing structures may be used inother examples of geophysical acquisition system according to theinvention.

The acquisition system shown in FIG. 1 can also include a plurality oflateral force and depth (“LFD”) control devices 26 cooperatively engagedwith each of the streamers 20 at selected positions along each streamer20. Each LFD control device 26 can include one or more rotatable controlsurfaces (not shown separately) that when moved to a selected rotaryorientation with respect to the direction of movement of such surfacesthrough the water 11 creates a hydrodynamic lift in a selected directionto urge the streamer 20 in any selected direction upward or downward inthe water 11 or laterally along the water surface with respect to thedirection of motion of the vessel 10. Thus, such LFD control devices 26can be used to maintain the streamers in a selected geometricarrangement. A non-limiting example of LFD device that may be used insome examples is described in U.S. Patent Application Publication No.2008/0008033 by Fossum et al. The particular configuration of the LFDdevices 26, however, is not a limit on the scope of the presentinvention. As previously explained, for purposes of the presentinvention it is only necessary for any devices used as the LFD devices26 be able to apply a selectable lateral force to the associatedstreamers 20. Depth control of the streamers 20 may be providedpassively, such as by providing the streamers 20 with a selected overallspecific gravity, or by separate depth control devices (not shown).Therefore, any reference to “depth” control as provided by the LFDdevices 26 is only intended to cover the present example implementation,such as using the device shown in the Fossum et al. '033 patentapplication publication referred to above. Any reference to active depthcontrol of the streamers 20 is not a limit on the scope of the presentinvention. For purposes of defining the scope of the invention,therefore, the LFD devices 26 need only perform the function of “lateralforce” control devices, and the inclusion of depth control as a part ofthe function of the LFD devices 26 explained herein is intended toensure that those of ordinary skill in the art understand that the useof the example LFD devices 26 disclosed herein, and any other similarexamples, are within the scope of the present invention.

In a particular implementation of the invention, a positiondetermination device may be associated with each LFD device 26. In oneexample, the position determination device may be an acoustic rangesensing device (“ARD”) 26A. Such ARDs typically include an ultrasonictransceiver or transmitter and electronic circuitry configured to causethe transceiver to emit pulses of acoustic energy. Travel time of theacoustic energy between a transmitter and a receiver disposed at aspaced apart position such as along the same streamer and/or on adifferent streamer, is related to the distance between the transmitterand a receiver, and the acoustic velocity of the water. The acousticvelocity can be assumed to not change substantially during a survey, orit can be measured by a device such as a water velocity test cell.Alternatively or additionally, acoustic range sensing devices (“ARDs”)may be disposed at selected positions along each one of the streamersnot collocated with the LFD devices 26. Such ARDs are shown at 23 inFIG. 1. Each of the ARDs 26A, 23 may be in signal communication with therecording system 12 such that at any moment in time the distance betweenany two ARDs 26A, 23 on any of the streamers 20 is determinable. One ormore ARDs may be placed at selected positions proximate the aft end ofthe vessel 10 so that relative distances between the selected positionson the vessel 10 and any of the ARDs on the streamers may also bedetermined. A non-limiting example of an ARD and system used with suchARDs is described in U.S. Pat. No. 7,376,045 issued to Falkenberg et al.and assigned to the assignee of the present invention.

The streamers 20 may additionally or alternatively include a pluralityof heading sensors 29 disposed at spaced apart positions along eachstreamer 20. The heading sensors 29 may be geomagnetic direction sensorssuch as magnetic compass devices affixed to the exterior of the streamer20. One type of compass device is described in U.S. Pat. No. 4,481,611issued to Burrage and incorporated herein by reference. The headingsensors 29 provide a signal indicative of the heading (direction withrespect to magnetic north) of the streamer 20 at the axial position ofthe heading sensor 29 along the respective streamer. Measurements ofsuch heading at spaced apart locations along each streamer may be usedto interpolate the geometry (spatial distribution) of each streamer.

Each streamer 20 may include at the distal end thereof a tail buoy 25.The tail buoy 25 may include, among other sensing devices, a geodeticposition signal receiver 25A such as a GPS receiver that can determinethe geodetic position of each tail buoy 25. The geodetic positionreceiver 25A in each tail buoy 25 may be in signal communication withthe recording system 12.

By determining the distance between ARDs 26A, 23, including the one ormore ARDs on the vessel 10, and/or by interpolating the spatialdistribution of the streamers from the heading sensor 29 measurements,an estimate of the geometry of each streamer 20 may be made.Collectively, the geometry of the streamers 20 may be referred to as the“array geometry.” For purposes of defining the scope of the presentinvention, the various position measurement components described above,including those from the heading sensors 29, from the ARDs 26A, 23, and,if used, from the additional geodetic position receivers 25A in the tailbuoys 25, may be used individually or in any combination. The ARDs andheading sensors may be referred to for convenience in defining theinvention as “relative position determination” sensors. By determiningrelative positions at each point along each streamer with reference to aselected point on the survey vessel or the energy source, is it possibleto determine the geodetic position of each such streamer point if thegeodetic position of the vessel or the energy source is determined. Asexplained above, the navigation portion of the recording system 12 mayinclude a GPS receiver or any other geodetic location receiver 12A. Insome examples, the energy source 17 may also include a geodetic positionlocation receiver 17A such as a GPS receiver. A particular example of asystem for determining relative positions of the streamers usingacoustic signals is described in the Falkenberg et al. patent referredto above.

During operation of the geophysical acquisition system shown in FIG. 1,it may be desirable to adjust portions of the streamers 20 laterally inorder to maintain a predetermined array geometry during geophysicalsurveying. The recording system 12 may be configured to send suitablecontrol signals to each of the LFD devices 26 to move associatedportions of each streamer 20 laterally. Such lateral motion may beselected so that each point along each streamer is located at apredetermined relative position at any moment in time. The relativepositions may be referenced to the position of either the survey vessel10 or the energy source 17. Examples of various array geometry controlmodes according to the invention are provided below.

During operation of the acquisition system shown in FIG. 1 when used forseismic surveying, for example, it is desirable for the streamers 20 tobe arranged as evenly as practicable behind the vessel 10 to avoid“holes” in the survey coverage. “Evenly” or “even” in the presentcontext means that the streamers 20 are substantially parallel to eachother along their entire length, that there is substantially equallateral distance between adjacent streamers, and that the streamersextend substantially parallel to a selected direction. Deviation fromsuch an even arrangement, as is known in the art, may be caused by ripcurrents in the body of water 11 and propeller wash from the surveyvessel 10, among other causes. “Holes” in the coverage is a conditionknown in the art wherein seismic sensors are disposed more sparsely thanwould be the case if the geometry of the array were “even” as definedabove.

1. Vessel Heading Follow Mode: Referring to FIG. 2, an automatictechnique to maintain a substantially even array geometry uses thevessel heading (geodetic direction) as a control parameter foradjustment of the array geometry. A sensor (not shown), such as amagnetic compass, in or associated with the recording system (12 inFIG. 1) measures the geodetic heading 42 of the survey vessel 10 andcommunicates such position to the recording system (12 in FIG. 1) atselected times according to operational programming of the recordingsystem (12 in FIG. 1). The measurements of geodetic heading 42 may befiltered.

A method according to the Vessel Heading Follow Mode includescommunicating control signals to each LFD device (26 in FIG. 1) suchthat the lateral distance of each streamer from each adjacent streameras measured by the ARDs (23 and 26A in FIG. 1) is maintained at aselected value, and the streamers 20 are maintained substantially in aselected geometry with respect to the vessel heading direction. Theselected geometry may be a straight line that is parallel to thedirection of the vessel heading. However, the selected geometry may be asubstantially straight line at an angle to the vessel heading direction.The selected geometry may be a streamer configuration in which thestreamers are maintained in a configuration other than a straight line.

In the present example, the recording system may generate controlsignals for each LFD device such that the lateral distance 30A from thecenterline 48 of the vessel measured by each of the ARDs is maintainedat a selected value along the length of each streamer 20 and thegeodetic position of any location along the streamers 20 follows thevessel heading trajectory.

The vessel heading 42 may differ from the vessel trajectory 40 becauseof the direction and magnitude of current flow 44 in the water 11. It isbelieved the vessel heading follow mode may have advantages underparticular environmental conditions, such as when a current flow 44 inthe water transverse to the towing direction varies in magnitude alongthe towing direction. An example of such transverse current would befound in an ocean lake or bay proximate the mouth of a river. Otherexamples will occur to those skilled in the art.

2. Source Trajectory Follow Mode: In another example, the trajectory 46of the energy source (17 in FIG. 1) may be used as the reference forcontrolling the array geometry. Referring to FIG. 3, an automatictechnique to maintain a substantially even array geometry uses thesource trajectory (geodetic direction) 46 as a control parameter foradjustment of the array geometry. A filtered value of the sourcetrajectory 46 is used as a reference to define the selected travel pathfor each streamer 20. The source trajectory 46 may be calculated in thenavigation portion of the recording system. Such calculation may includemaking a record with respect to time of the geodetic position of thesource 17 within selected time intervals. The calculation programminginstructions may include a smoothing filter for the record of sourceposition with respect to time. In the present example, the recordingsystem may generate control signals for each LFD device such that thelateral distance 30 with respect to the source trajectory 46 measured byeach of the ARDs is maintained at a selected value and/or the geodetictrajectory of any location along the streamers 20 follows the sourcetrajectory.

A method according to the Source Trajectory Follow Mode includescommunicating control signals to each LFD device (26 in FIG. 1) suchthat the lateral distance of each streamer from each adjacent streameras measured by the ARDs (23 and 26A in FIG. 1) is maintained in aselected geometry with respect to the source trajectory direction. Theselected geometry may be substantially a straight line that is parallelto the vessel trajectory direction. However, the selected geometry maybe a substantially straight line at an angle to the vessel trajectorydirection. The selected geometry may be a streamer configuration inwhich the streamers are maintained in a configuration other than astraight line.

3. Preplot Follow Mode: A seismic survey “preplot” line (a geodetic pathdefined by the subsurface area being investigated by the seismic survey)can also be used as a control variable for the selected travel path ofeach streamer. In the preplot follow mode, reference to the geodeticposition and/or heading of the vessel or the source may be omitted. Thearray geometry may be configured to move the geophysical sensors along apath based only on the geodetic path intended to be followed by thesensors in maling measurements of the subsurface. In one example, apreplot survey path may be predetermined for a geophysical survey of anarea of the subsurface, and for subsequent surveys conducted over thesame area, the same preplot survey path may be utilized for conductingthe subsequent geophysical surveys. In another example, subsequentgeophysical surveys may be conducted over substantially the same area ofthe subsurface as a previous survey, and the recorded locations of thesensors from the previous survey are utilized for guidance in selectingthe travel path for each streamer.

The preplot follow mode may have advantages in “4D” seismic surveying,wherein a survey is repeatedly conducted over a same area of thesubsurface at selected times. As is known in the art, the accuracy ofsuch 4D surveys may be increased by causing the geophysical sensors toas closely as possible follow a same geodetic path each time a survey isconducted. The preplot follow mode may improve the capability of thesensor array to make such repeated travel paths.

The preplot follow mode may be performed using geodetic position signalsdetected, for example, using the geodetic position signal receivers (25Ain FIG. 1) on the tail buoys (25 in FIG. 1) to determine geodeticposition of at least one point in the array. Relative positions betweenthe sensors in the array may be determined as explained above using ARDs(23 and 26A in FIG. 1). Geodetic heading or direction of the streamersmay be determined, for example, using the heading sensors (29 in FIG. 1)as explained above. The LFDs (26 in FIG. 1) may be operated such thatthe streamers follow a selected geodetic path substantially irrespectiveof the vessel position and the vessel heading. It will be appreciated bythose skilled in the art that the possible range of array geometry maybe limited by the lengths of the various towing components and theoffset of the vessel trajectory from the centerline of the predeterminedarray travel path, however the preplot follow mode may be used toprovide more consistent survey results between successive surveys of thesame subsurface geodetic area than methods that control array geometryonly by reference to the vessel trajectory or heading.

4. Proportional Force Separation Mode: During deployment and retrievalof the streamers from the vessel, and during periods of severe weather,the focus of the steering of seismic streamers is not on subsurfacecoverage, but on increasing the safety of the streamers fromentanglement with each other. One way to obtain a high degree of safetyis to apply a selected lateral force (or constant LFD steering elementdeflection angles) on each streamer in a direction outward from thecenterline of the vessel, with the largest lateral force being appliedon the laterally outermost streamers, and with a linear or otherproportional relationship between the lateral force being applied oneach streamer and the lateral distance from the centerline of the vesselof each respective streamer. As an example, for a 10 streamer array, therightmost streamer (designated streamer 1) could apply a 400N (or, forexample, a 15 degree “wing angle” on the LFD device) lateral forceoutward from the centerline of the vessel, and lateral outward forces of320N, 240N, 160N and 80N, respectively, on streamers 2, 3, 4 and 5. Thesame lateral force values could be applied for streamers 6 through 10,but in the opposite direction from the centerline of the vessel. Theforegoing steering principle is illustrated in FIG. 4, where lateralforce F1 and F3 are smaller than corresponding lateral forces F2 and F4.The term “proportional” as used herein is intended to mean asubstantially monotonically increasing force with respect to lateraldistance from the centerline of the vessel and is not limited to alinear or other directly proportional relationship.

By providing such lateral outward forces on each streamer, a sufficientlateral separation of the streamers can be achieved even without theneed for high quality position information, or even with no positioninginformation, whether relative or absolute. To further increase thesafety of deployment and retrieval operations, the depth of eachstreamer could be related to lateral distance from the centerline of thevessel, such that separation of the streamers may be maintained in twoplanes.

It might sometimes be desirable to steer all devices in one direction.The principle then is as above, but with the outmost streamer on oneside applying a large force and with a reduction of force for everystreamer until zero or a small force is applied on the outermoststreamer on the other side. The main principle of such mode is that anindividually selected, constant force is applied to the steering devicesto optimize streamer control.

Methods for operating LFD devices and controlling geometry of a sensorarray according to the various aspects of the invention may provide moreeven coverage in marine geophysical surveying, may provide more accuratepositioning of geophysical sensors, and may improve safety of the arrayin hostile environments.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method for towing a sensor streamer array in a body of water,comprising: towing a plurality of sensor streamers behind a surveyvessel in the water; determining a relative position at selected pointsalong each streamer with respect to the vessel; and laterally deflectingat least one streamer at at least one longitudinal position therealongin response to the determined positions such that the streamers aremaintained in a selected geometry in the water, in which the determinedpositions along each streamer substantially follow a trajectory ofgeodetic position of a geophysical energy source towed in the water. 2.The method of claim 1 wherein the energy source is towed by the surveyvessel.
 3. The method of claim 1 wherein the determining relativeposition comprises measuring acoustic travel time to a selected positionalong another streamer.
 4. The method of claim 1 wherein the determiningrelative position comprises measuring geomagnetic direction.
 5. Themethod of claim 1 wherein the sensor streamers comprise seismic sensors.6. The method of claim 1 wherein the sensor streamers compriseelectromagnetic sensors.
 7. A method for towing a sensor streamer arrayin a body of water, comprising: towing a plurality of sensor streamersbehind a survey vessel in the water; determining a relative position atselected points along each streamer with respect to the vessel; andlaterally deflecting at least one streamer at at least one longitudinalposition therealong in response to the determined positions such thatthe streamers are maintained in a selected geometry in the water, inwhich the determined positions along each streamer substantially followa geodetic path of sensors along a selected path.
 8. The method of claim7 wherein the determining relative position comprises measuring acoustictravel time to a selected position along another streamer.
 9. The methodof claim 7 wherein the determining relative position comprises measuringgeomagnetic direction.
 10. The method of claim 7 wherein the sensorstreamers comprise seismic sensors.
 11. The method of claim 7 whereinthe sensor streamers comprise electromagnetic sensors.
 12. A method fortowing a sensor streamer array in a body of water, comprising: towing aplurality of sensor streamers behind a survey vessel in the water;determining a relative position at selected points along each streamerwith respect to the vessel; and laterally deflecting at least onestreamer at at least one longitudinal position therealong in response tothe determined positions such that the streamers are maintained in aselected geometry in the water, in which the determined positions alongeach streamer substantially follow a geodetic heading of the surveyvessel.
 13. The method of claim 12 wherein the determining relativeposition comprises measuring acoustic travel time to a selected positionalong another streamer.
 14. The method of claim 12 wherein thedetermining relative position comprises measuring geomagnetic direction.15. The method of claim 12 wherein the sensor streamers comprise seismicsensors.
 16. The method of claim 12 wherein the sensor streamerscomprise electromagnetic sensors.
 17. The method of claim 12 wherein thegeodetic heading is a geomagnetic heading.
 18. A method for towing asensor streamer array in a body of water, comprising: towing a pluralityof sensor streamers behind a survey vessel in the water; applying alateral deflecting force at selected positions along each streamer, thelateral deflecting force at each selected position related to a lateraldistance of a forward end of each streamer from a center line of thesurvey vessel.